The present invention relates to a color image forming apparatus provided with a heat roller type fixing device in which a fixing roller is heated when heat is supplied from a pressure roller.
In general, the heat roller type fixing device includes a fixing roller in which a heater is provided and a pressure roller, the circumferential surface of which is covered with heat-resistant rubber. However, the fixing device of the present invention includes a fixing roller having no heater, the circumference of which is covered with resilient heat-resistant rubber, and also includes a metallic pressure roller in which a heater is provided, wherein the fixing roller and pressure roller are combined to carry out a fixing operation. In this fixing device, toner on a transfer sheet is heated and fixed by the fixing roller heated by the action of the pressure roller having a heater.
When an electrical power switch of the image forming apparatus is turned on, the pressure roller is rotated and the heater is energized. When the temperature of the circumferential surface has reached a predetermined setting value t.sub.0 as shown in FIG. 4(a), electrical power supply to the heater is repeatedly turned on and off so as to maintain setting temperature t.sub.0.
On the other hand, the fixing roller receives heat from the circumferential surface of the pressure roller, and the circumferential temperature of the fixing roller is gradually raised as shown by a solid line in FIG. 4(b). When the temperature has been increased to the predetermined fixing temperature t.sub.1, or t.sub.2, a control section sends a signal of indicating a completion of the warming-up, and the apparatus displays a condition of ready-to-copy.
However, the result of detection by a temperature sensor to detect the circumferential temperature of the fixing roller fluctuates a little below the circumferential temperature of the fixing roller as shown by a broken line in FIG. 4(b) because the heat capacity of the temperature sensor affects the response of detection and the deviation of the detected temperature.
For example, in the case where transfer sheets are thin and relatively low fixing temperature t.sub.1 is set as the circumferential temperature of the fixing roller, time lag T.sub.1 caused by the temperature sensor is very small. However, in the case where transfer sheets are thick and high fixing temperature t.sub.2 is set, time lag T.sub.2 is considerable.
Fixing temperature to be set at the fixing roller depends on the thickness of a transfer sheet, the size of the transfer sheet, and the color of toner to be fixed, and further the fixing temperature depends on the required surface condition of an image, that is, whether the image is glossy or nonglossy. In the case where a high fixing temperature is set, an unnecessarily long period of time is required for warming-up, that is, a long period of time passes until ready-to-copy is displayed.